The present invention relates to a read-only optical disk for conducting optical reading.
Read-only optical disks have generally been put to practical use as low cost and large capacity recording media.
A read-only optical disk generally comprises a transparent substrate having pits (minute recesses and protrusions) corresponding to recording signals formed on one of the surfaces thereof, on which a reflective layer (usually made of Al) is provided and a protective resin coating is applied further thereover.
However, the optical disk having this structure has a problem in view of corrosion resistance. For instance, peeling of the reflective layer caused by expansion and shrinkage of the substrate when undergoing abrupt change of temperature and humidity, corrosion of the reflective layer caused under high temperature and humidity, etc. directly lead to increased error rate, thereby greatly deteriorating the reliability of the optical disk.
In particular, if a disk is used in a special circumstance, such as under high temperature or low temperature, the problem of the corrosion resistance as described above becomes extremely significant.
In view of the foregoing problems, the present inventors have made an earnest study for providing a read-only optical disk having high corrosion resistance and, as a result, have found that a disk of excellent corrosion resistance can be obtained by selecting a specific structure. The present invention has been accomplished based on this finding.